Field of the Invention
The present invention relates to an autofocus (“AF”) apparatus and an optical apparatus.
Description of the Related Art
A contrast focus detecting (contrast AF) method is known as a focus detecting method for extracting a spatial frequency of a high-frequency component from an image signal obtained from an image sensor and for detecting, as an in-focus position, a peak position of an AF evaluation value that is a contrast value of the spatial frequency. In general, the peak position of the AF evaluation value is different according to spatial frequencies of objects due to the aberration of an image pickup optical system. An object having a higher spatial frequency is more sensitive to defocus, and the defocus is likely to appear as an image blur (image quality drop). The contrast AF thus needs to detect an in-focus position using a frequency component that is as high as possible. However, a higher frequency component to be extracted is more likely to be affected by noises etc. Then, the detecting reliability of this frequency component is likely to lower, and the AF accuracy or image quality is likely to deteriorate.
Japanese Patent No. (“JP”) 5,106,146 proposes an AF apparatus that sets one of a plurality of filters each used to extract a high-frequency component, based on the illuminance in image capturing, and moves a focus lens from an obtained in-focus position to a position that shifts by a predetermined amount determined by characteristic information of image pickup optical system. JP 5,164,493 discloses a method for moving the focus lens by the predetermined amount in a defocus direction for the center of the image and in a focus improving direction for the periphery of the image.
Due to the recent demands for a smaller size, a thinner profile, and a larger aperture diameter of the image pickup apparatus, a wider image circle, and a higher magnification of a zoom lens, it becomes difficult to restrain the aberration of the image pickup optical system and the asymmetry with respect to the peak position increases as the spatial frequency of the object becomes higher. The AF evaluation value remarkably drops with a slight defocus on the steep slope side of the asymmetry. The predetermined amounts in JPs 5,106,146 and 5,164,493 are affected by errors, such as a manufacturing error (individual difference) of each lens in the image pickup optical system, an assembly error, etc. Hence, in moving the focus lens according to the teachings in JPs 5,106,146 and 5,164,493, the image quality may drastically deteriorate on the steep slope side of the asymmetry.